Energy is conserved during a chemical reaction. A chemical reaction is fundamentally a process where atoms in reactant molecules reorganize to form new product molecules. This rearrangement results in a change of substances, but the total energy associated with the atoms and their connections is not lost or gained overall. Understanding this truth is foundational to the study of chemistry and allows scientists to predict the energy flow in any chemical process.
The Governing Principle: The Law of Conservation
The theoretical backbone for energy changes in all chemical processes is the Law of Conservation of Energy, also referred to as the First Law of Thermodynamics. This principle states that energy can never be created or destroyed; it can only be converted from one form into another, such as from chemical potential energy to heat or light energy.
To apply this law, chemists divide the universe into two parts: the system and the surroundings. The system is the specific chemical reaction being studied, which includes the reacting molecules and their products. The surroundings are everything else in the universe that is not part of the reaction, such as the container, the air, and anything else nearby.
Any energy change that occurs within the system must be balanced by an equal and opposite change in the surroundings. If the system loses energy, that exact amount must be transferred to the surroundings, often making them warmer. Conversely, if the system gains energy, it must absorb it from the surroundings, causing them to cool down.
Energy Storage and Transfer Through Chemical Bonds
The energy that is conserved during a reaction is initially stored as chemical potential energy within the bonds that hold atoms together in a molecule. When a reaction begins, the first step requires an input of energy to break the existing chemical bonds in the reactant molecules. Breaking these bonds is an energy-absorbing process, similar to the effort required to pull two magnets apart.
Once the old bonds are broken, the atoms are free to rearrange, and new chemical bonds form to create the product molecules. The formation of these new, more stable bonds releases energy back into the system and its surroundings. The energy difference between the amount absorbed to break the old bonds and the amount released by forming the new bonds dictates the net energy change of the entire reaction.
This net change is known as the enthalpy change (Delta H), which represents the difference in chemical potential energy between the reactants and the products. If the energy released by forming the new bonds is greater than the energy required to break the old bonds, the excess energy is released from the system. If the energy required to break the bonds is greater than the energy released by forming new ones, the system must draw energy from the surroundings to complete the process.
Observable Energy Dynamics: Exothermic and Endothermic Reactions
The principle of energy conservation becomes observable through the two primary types of chemical reactions: exothermic and endothermic. Exothermic reactions are defined by a net release of energy from the system into the surroundings, usually in the form of heat or light. Combustion, such as burning natural gas, is a common example where the energy released from forming strong product bonds exceeds the energy needed to break the reactant bonds. This excess energy enters the surroundings, causing the temperature to rise.
Endothermic reactions, conversely, involve a net absorption of energy from the surroundings into the reaction system. In these cases, the energy required to break the reactant bonds is greater than the energy released by forming the product bonds. The system must draw the deficit of energy from its environment, which typically causes the surrounding area to feel cold. A practical example is the chemical reaction inside an instant cold pack, where dissolving a salt in water absorbs heat.
These visible changes in temperature or the production of light are simply the outward manifestations of the initial chemical potential energy being reallocated. In an exothermic reaction, the chemical energy is converted and expelled as thermal energy. In an endothermic reaction, thermal energy from the surroundings is converted into new chemical potential energy stored in the product molecules.